1. Field of the Invention
The invention relates to a method for sintering oxidic nuclear fuel blanks into oxidic nuclear fuel pellets, in which the blanks are moved through the muffle of a tunnel furnace.
2. Description of the Prior Art
Transport boats have been used heretofore, on which a large number of blanks were stacked side by side and on top of each other, for transporting the blanks through the muffle of the tunnel furnace. Because of this stacking of the blanks, the temperature and the action of gases in the furnace are of necessity not identical for each blank, with the result that the deviation of the characteristics from the standard particularly of the dimensions and the density, of the sintered fuel pellets is quite large such that the pellets must be measured after the sintering and in part reworked by grinding.
The known method employing transport boats further requires a large cross section of the muffle, which means a correspondingly large material and power requirement; the latter is additionally increased because the transport boats must also be heated.
A further disadvantage of the known method involves the necessity of changing the process parameters, especially the residence time of the blanks, the temperature profile and the partial oxygen pressure to obtain pellet products of desired characteristics. This can be determined only after a transport boat has been unloaded and the fuel pellets have been measured; in the meantime, however, the incorrectly adjusted process continues to run and produce rejects.
Also, the latitude for process parameters is small with the known method since it is limited by the material properties of the transport boats which are subjected to the same process parameters as the blanks.
Thus, for instance, the temperature in the low-temperature short-time sintering process disclosed in German DE-C 28 55 166 and corresponding to U.S. Pat. No. 4,348,339, had to be limited because of its effect on the material for the transport boats in an oxidizing and reducing atmosphere.
Also, in oxidatively reducing sintering according to German DE-C 29 39 415, corresponding to allowed U.S. application Ser. No. 190,981 filed Sept. 26, 1980, now U.S. Pat. No. 4,438,050, the transport boats would have to withstand temperatures up to 1500.degree. C. with different oxygen activities with a reduction in economy as a result thereof in large scale operation.